246 Forestry Quarterly. 



cur in warmer weather, caused either by wind or sun. If caused 

 by a chinook a ground avalanche may occur by night in any lo- 

 caHty, because of the warm wind blowing. If caused by the sun 

 it is likely to occur on a south slope during the early evening. 

 The Italian language expresses this very correctly by calling 

 a dust avalanche a "cold avalanche" (avalanga fredda) and a 

 ground avalanche a warm avalanche (avalanga colda). 



The statistics show that 60% of the avalanches start on 

 rocky ground, and only 6% in timber or from brushy places. It 

 appears, therefore, that the soil cover has a great influence on 

 the origin of avalanches. The taller the trees and the less they 

 are swayed by the wind near the ground, the closer does the 

 snow hug the ground. In a closed stand of timber the formation 

 of an avalanche is impossible. The higher the altitude, the 

 closer do the shrubby species grow to the heat-reflecting soil. 

 The alder is usually the first species which covers the slideways 

 of avalanches. These alder brakes can be seen for a long distance 

 on places where slides occur frequently. The geological forma- 

 tion of the rocks has also an influence on the origin of ava- 

 lanches. Stratified rocks, like slate, increase the number, particu- 

 larly if the strata are more or less parallel to surface of the slope ; 

 similarly smooth rocks without soil cover or brush protection, and 

 especially if water from many small springs seeps through the 

 soil. The safest are igneous rocks, as basalt or granite. 



Other causes which start avalanches are : ( i ) Movements of 

 loose snow, or steep slopes; (2) Game or men: (3) Earthquakes, 

 blasting, trains, shots, or even the ringing of church bells, echoes ; 

 (4) Wind, breaking off of rocks, of over-hanging snow drifts, 

 of icicles and limbs; (5) Heat from sun, reflection of heat from 

 rock walls; (6)Extreme changes from warm and wet to cold 

 weather, or vice versa. The range of latitude in the Swiss 

 Alps, compared with that of our mountain regions, is very 

 small, and the data under the sub-heading "Absolute Elevation 

 of the Starting Point" relate only to that confined region. 



During glacial times there seem to have been practically no 

 avalanches, but they increased during the post-glacial period, and 

 reached their climax before vegetation was established on these 

 old glacial fields. It is interesting from the ecological point 

 of view that in the Alps a Rhododendron and a Boxelder have 



